Dispersion-shifted monomode optical fiber

ABSTRACT

Dispersion-shifted monomode optical fibers have an effective mode surface area greater than 65 μm 2  by optimization of the geometrical characteristics that characterize the fibers. The fibers have substantially zero chromatic dispersion in the vicinity of 1.55 μm, and they include an optical core having a central portion, a first layer having an index lower than the index of the central portion, and a second layer having an index higher than the index of the first layer and higher than the index of the optical cladding.

The present invention relates to a dispersion-shifted monomode opticalfiber.

BACKGROUND OF THE INVENTION

So-called "dispersion-shifted" monomode optical fibers are such that atthe transmission wavelength at which they are used, which is generallyother than 1.3 μm (the wavelength at which the dispersion of silica issubstantially zero), the chromatic dispersion of the transmitted wave issubstantially zero, i.e. the non-zero chromatic dispersion of the silicais compensated (hence the use of the term "offset") in particular by anincrease in the index difference Δn between the core and the opticalcladding.

The transmission wavelength presently selected for line fibers, i.e.fibers designed to perform long distance transmission, e.g. fortransoceanic connections, is substantially equal to 1.55 μm. It is atthis wavelength that it is possible to obtain minimum transmissionattenuation of light, of the order of 0.2 dB/km.

Thus, in the context of the present invention, the fibers underconsideration are designed to be used at a wavelength of 1.55 μm sincethat is the most efficient for transmission.

Also, it is well known that the bandwidth of monomode optical fibers ismuch greater than that of multimode fibers, which is why present andfuture developments of lines for long distance transmission concentrateon monomode optical fibers.

Consequently, the present invention applies most particularly todispersion-shifted monomode optical fibers designed to be used at awavelength substantially equal to 1.55 μm.

More precisely, the invention relates to such optical fibers in whichcurvature losses do not exceed 0.005 dB/m when the radius of curvatureis 30 mm. It is well known that such a limitation on curvature losses isnecessary to ensure that the optical fiber operates under propertransmission conditions.

At present, numerous dispersion-shifted monomode optical fiber profilesare being studied and they are widely described in the literature.

The simplest known profiles referred to as "step", "trapezium", or"triangle", are such that the refractive index in the core varies as afunction of distance from the axis of the fiber, so that when shown as afunction of said distance the index appears as a curve constitutingrespectively a rectangle, a trapezium, or a triangle, while the index inthe optical cladding surrounding the core is constant and less than thatof the core.

A "pedestal" profile is also known in which the central portion formingthe "inner" core of the optical fiber is surrounded successively by an"outer" core of refractive index lower than that of the inner core, andthen by optical cladding of index lower than that of the outer core.

Also known is a profile referred to as being of the "trapezium andcentral ring" type which is shown very diagrammatically in FIG. 1, wherethere can be seen the curve representing the refractive index n in thefiber as a function of distance d from the axis of the fiber. In thatprofile, the core C comprises:

a central portion 10 having a maximum index n_(s) +Δn in which the indexvaries in such a manner as to give the curve the form of a trapezium,and in the limit, of a triangle or of a rectangle;

a layer 11 of index n_(s), e.g. constant and less than n_(s) +Δn,surrounding the central portion 10; and

a layer 12 surrounding the layer 11 and of index n_(s) +hΔn (0<h<1),which is constant for example, greater than n_(s), and less than n_(s)+Δn.

The layer 12 is surrounded by a cladding layer G of index equal ton_(s).

In practice, the term "trapezium" when used for the central portion 10of the core C covers the limiting shapes of a triangle and of arectangle.

Finally, as described in an article entitled "Transmissioncharacteristics of a coaxial optical fiber line", published in Journalof Lightwave Technology, Vol. 11, No. 11, November 1993, a profile isknown of the "buried central hollow" type which is shown verydiagrammatically in FIG. 2, where there can be seen the curve ofrefractive index n in the optical fiber as a function of distance d fromthe axis of the fiber. In that profile, the core C' comprises a centralportion 20 of minimum index n_(s) +hΔn (h<0) surrounded by a layer 21 ofindex n_(s) +Δn greater than n_(s) +hΔn. The layer 21 is surrounded by acladding layer G' of index equal to n_(s).

It is recalled that all of the above-mentioned profiles are naturallycircularly symmetrical about the axis of the optical fiber.

All of those profiles make it possible to obtain substantially zerochromatic dispersion at 1.55 μm, while also obtaining low attenuationand curvature losses. Nevertheless, a constant concern in the context ofdeveloping long distance links using optical fibers is that of furtherimproving transmission quality and of reducing the cost thereof.

Transmission quality is associated with the signal-to-noise ratio alongthe link, with noise coming from amplified spontaneous emissiongenerated by the amplifiers belonging to the repeaters used along thetransmission line, and it has been shown that this signal-to-noise ratiois itself inversely proportional to a "penalty" function F of the fiberwhich depends on the distance Z between amplifiers, on the effectivemode surface area S_(eff) of the optical fiber used, on the populationinversion factor n_(sp), on the linear attenuation α, and on thecoupling coefficients C₁ and C₂ respectively at the inlet and at theoutlet of an amplifier. The penalty function F is thus given by theformula: ##EQU1##

From that formula, it will be understood that to improve transmissionquality, attempts can be made:

to reduce the population inversion factor n_(sp), leaving other thingsequal; nevertheless that requires complex development with respect topumping wavelength and thus concerning line components other than theoptical fiber;

to reduce the attenuation α; nevertheless since attenuation is alreadyvery low at 1.55 μm (in practice around 0.2 dB/km), any reduction thatcan be hoped for will have little influence on the penalty function F;

to act on the coupling coefficients C₁ and C₂ ; that also requiresaction to be taken on line components other than the optical fiber andtherefore requires complex development; and/or

to increase the effective mode surface area S_(eff) ; that does indeedmake it possible to improve the quality of the link.

FIG. 3 shows the penalty function F in dB for an optical fiber usingsoliton type transmission and plotted as a function of the distance Z inkm between amplifiers for a known optical fiber having an effective modesurface area of 50 μm² (curve 30) and for a desirable optical fiberhaving an effective mode surface area of 70 μm² (curve 31), with all theother parameters on which F depends being given and remaining unchanged.It can be seen that for given penalty function, i.e. givensignal-to-noise ratio, the greater the effective mode surface area, thegreater the distance between amplifiers, thus making it possible toreduce the number of repeaters used, and hence reduce the cost of thesystem.

Also, it can be seen that for given distance between amplifiers, thegreater the effective mode surface area, the smaller the penaltyfunction, i.e. the better the quality of transmission.

Hence, to improve transmission quality, or indeed in equivalent manner,to reduce the number of repeaters used for given quality of the link,thus making it possible to reduce the cost of the link, it isadvantageous to increase the effective mode surface area.

With simpler index profiles, such as the step, trapezium, or triangleprofiles, in order to obtain substantially zero chromatic dispersion at1.55 μm, i.e. to compensate for the chromatic dispersion of silica at1.55 μm, it is necessary to increase the index difference between thecore and the cladding, thereby necessarily giving rise to a decrease inthe effective mode surface area.

Thus, to obtain large effective mode surface areas while ensuringsubstantially zero chromatic dispersion at 1.55 μm, it is necessary toopt for more complex index profiles such as the profiles shown in FIGS.1 and 2.

Until now, studies performed on the trapezium and central ring typeprofile have led to effective mode surface areas that do not exceed 50μm² to 60 μm². So far no study has been performed on the buried centralhollow profile enabling an effective mode surface area to be determined.

OBJECT AND SUMMARY OF THE INVENTION

The object of the present invention is thus to optimize the geometricalparameters of known profiles to make it possible to obtain effectivemode surface areas greater than those obtained with conventionalprofiles, so as to make monomode optical fibers of zero chromaticdispersion in the vicinity of 1.55 μm while also having an effectivemode surface area greater than 65 μm², and while maintaining attenuationand curvature losses equivalent to those obtained with known opticalfibers.

To this end, the present invention provides a monomode optical fiberhaving substantially zero chromatic dispersion in the vicinity of 1.55μm, comprising an optical core having:

a central portion whose refractive index as a function of distance fromthe axis of said fiber varies between a minimum index n_(s) and amaximum index n_(s) +Δn, where Δn is strictly positive, said index beingrepresented by a curve that is substantially trapezium shaped;

a first layer surrounding said central portion, having an indexsubstantially equal to n_(s) ; and

a second layer surrounding said first layer, having an index varyingover the range n_(s) to n_(s) +hΔn, where 0<h<1;

and optical cladding surrounding said second layer and having an indexsubstantially equal to n_(s), said profile being defined by thefollowing geometrical parameters:

a: total radius of the core, measured at said second layer;

x: ratio of the radius of said central portion to a, where 0<x<1;

y: ratio of the radius of said first layer to a, where x<y<1; and

r: ratio of the small base to the large base of said trapezium, with0≦r≦1;

said fiber being characterized in that a and Δn are determined so thatthe chromatic dispersion of said fiber is substantially zero at 1.55 μmand the cutoff wavelength λ_(c) of said fiber is such that: 1.4 μm<λ_(c)<1.55 μm, r being selected arbitrarily in the range 0 to 1, and then x,y, and h being selected so as to satisfy the following relationships:##EQU2##

In practice, relationship 3) sets limit values for the ratio of thesurface area defined by the curve giving n as a function of d for thecentral ring of the profile over the surface area defined by the curvegiving n as a function of d for the trapezium.

With the optical fiber of the invention having the trapezium and centralring type profile, effective mode surface areas are obtained that aregreater than 65 μm², extending up to 85 μm². For given quality of alink, this makes it possible to increase the distance between amplifiersby 10% to 30%.

The present invention also proposes, to solve the problem posed, amonomode optical fibers with substantially zero chromatic dispersion inthe vicinity of 1.55 μm, comprising an optical core having:

a central portion of index varying with distance from the axis of saidoptical fiber between a minimum index n_(s) +hΔn, with -1<h<0 and withΔn strictly positive, and a maximum index n_(s) ;

a layer surrounding said central portion, of index that varies withdistance from the axis of said fiber over the range n_(s) to n_(s) +Δn;

and optical cladding surrounding said layer and having an indexsubstantially equal to n_(s), said profile being defined by thefollowing geometrical parameters:

a: total radius of the core measured at said layer;

y: ratio of the radius of said central portion to a, where 0<y<1;

said fiber being characterized in that a and Δn are determined so thatthe chromatic dispersion of said fiber is substantially zero at 1.55 μmand the cutoff wavelength λ_(c) of said fiber is such that 1.4 μm<λ_(c)=1.55 μm, y being an arbitrary value in the range 0 to 1, and h beingselected so that the following relationships are satisfied:

    h.sub.1 (y)≦h≦h.sub.2 (y)                    1)

where: ##EQU3##

    -2.33y+0.6<h<-0.2.                                         2)

With the optical fiber of the invention having a profile of the buriedcentral hollow type, effective mode surface areas are obtained that aregreater than 65 μm², and that extend up to 95 μm². This likewise makesit possible, for given quality of the link, to increase the distancebetween amplifiers by 10% to 40%.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention appearfrom the following description of implementations of the presentinvention, given by way of non-limiting illustration.

In the following figures:

FIG. 1 shows variations in the refractive index n as a function ofdistance d from the axis of the optical fiber for various layers of amonomode fiber having a profile of the trapezium and central ring type;

FIG. 2 shows variations in the refractive index n as a function ofdistance d from the axis of the optical fiber for various layers of amonomode fiber having a profile of the buried central hollow type;

FIG. 3 shows the penalty function F as a function of distance Z betweenamplifiers for a monomode optical fiber having substantially zerochromatic dispersion at 1.55 μm and using soliton type transmission; and

FIG. 4 shows variations in the ideal and real refractive indices n as afunction of distance d from the axis of the optical fiber for variouslayers of a monomode optical fiber having a profile of the trapezium andcentral ring type.

MORE DETAILED DESCRIPTION

FIGS. 1 to 3 are described above in the description of the state of theart.

As specified above, the known profiles of the trapezium and central ringtype and of the buried central hollow type make it possible, byselecting their geometrical parameters in application of the criteria ofthe present invention, to obtain effective mode surface areas that areconsiderably greater than those obtained either with conventionalprofiles of the step, trapezium, or triangle type, or with trapezium andcentral ring or buried central hollow type profiles as implemented inthe prior art. The present invention thus makes it possible to optimizethe selection of geometrical parameters for these profiles so as tosatisfy requirements in terms of effective mode surface area, chromaticdispersion, attenuation, and curvature losses.

Three possible implementations of the present invention are describedbelow as examples, giving values for the above-defined geometricalparameters and satisfying the relationships of the invention, and alsogiving the effective mode surface area, the chromatic dispersion at 1.55μm, the cutoff wavelength, the attenuation, and the curvature lossesobtained with profiles possessing these characteristics.

Initially it is recalled that all of the geometrical characteristics ofthe profiles of the invention are functions of two basic parameters: thetotal radius a of the fiber core, and the index difference Δn betweenthe maximum index of the core and the index of the cladding.

These fundamental parameters can be determined in conventional manner soas to satisfy the essential requirements for optical fibers in thecontext of the present invention, these requirements being thefollowing:

substantially zero chromatic dispersion, i.e., in practice, less than 1ps/(nm.km) in the vicinity of 1.55 μm;

a cutoff wavelength λ_(c) such that: 1.4 μm<λ_(c) <1.55 μm to ensurethat transmission is monomode at the desired wavelengths and to reducecurvature losses; and

attenuation close to 0.2 dB/km.

It is briefly recalled how the parameters a and Δn can be selected tosatisfy the above criteria for a profile of the trapezium and centralring type, with the reasoning applying in similar manner to a profile ofthe buried central hollow type.

In known manner, chromatic dispersion C can be put in the form of afunction of wavelength λ as follows: ##EQU4## where: M(λ) is a knownterm characterizing the chromatic dispersion of pure silica atwavelength λ(M(λ) is about 22 ps/(nn.km) at 1.55 μm); ##EQU5## is a termcharacterizing the dispersion of the waveguide, V being normalizedfrequency and B being normalized effective index (it is recalled thatthe effective index is the index as effectively "seen" by the lightwavepropagating in the core), which is a function of V;

c is the speed of light in a vacuum; and

ε(λ) is a negligible term.

Given that it is a desire to have a cutoff wavelength λ_(c) such thatguidance is monomode in the range 1.4 μm to 1.55 μm, a range can bededuced therefrom defining variations in the normalized frequency V, andthus a corresponding range for B. It is therefore possible to deduce Δnof equation (1) and then a of equation (2) which gives the normalizedfrequency: ##EQU6## n_(s) being the index of the cladding and λ beingthe operating wavelength.

In general, for optical fibers having a profile of the trapezium andcentral ring type, in order to satisfy the above conditions, the coreradius a can be selected to lie in the range 2 μm to 9 μm, and the indexdifference Δn to lie in the range 8×10⁻³ to 20×10⁻³.

For optical fibers having a profile of the buried central hollow type,to satisfy the above conditions, the core radius a can be selected tolie in the range 2.5 μm to 4 μm, and the index difference Δn to lie inthe range 12×10⁻³ to 20×10⁻³.

Also, for the trapezium and ring type profile, the value of r can beselected arbitrarily in the range 0 to 1, and it will be understood thatthe term "trapezium" should be interpreted broadly in the context of thepresent invention, specifically as including the limiting cases where ris equal to 0 (the trapezium is then a triangle) or equal to 1 (thetrapezium is then a rectangle).

The same applies to the value X for the profile of the buried centralhollow type, where X can be selected arbitrarily between 0 and 1 in thestrict sense.

EXAMPLE 1

In this example, the optical fiber is of the trapezium and central ringtype. The values of the various characteristic parameters of the fiberare as follows:

a=6.97 μm

Δn=10×10⁻³

r=0.6

x=0.35

y=0.69

h=0.325.

With such a fiber, chromatic dispersion is 0.685 ps/(nm.km) at 1558 nm,the cutoff wavelength is 1.48 μm, and attenuation is 0.20 dB/km.Curvature losses are less than 0.005 dB/m, for a radius of curvature of30 mm.

The effective mode surface area is 71 μm².

EXAMPLE 2

In this example, the optical fiber is of the buried central hollow type.The values of the various characteristic parameters of the fiber are asfollows:

a=3.07 μm

Δn=15.6×10⁻³

y=0.56

h=0.55.

With such a fiber, chromatic dispersion is 0.7 ps/(nm.km) at 1558 nm,the cutoff wavelength is 1.485 μm, and attenuation is about 0.21 dB/km.Curvature losses are less than 5×10⁻⁷ dB/m.

The effective mode surface area is 71.9 μm².

EXAMPLE 3

In this example, the optical fiber is of the buried central hollow type.The values of the various characteristic parameters of the fiber are asfollows:

a=3.3 μm

Δn=14.7×10⁻³

y=0.606

h=0.599.

With such a fiber, chromatic dispersion is 0.7 ps/(nm.km) at 1558 nm,the cutoff wavelength is 1.485 μm, and attenuation is about 0.21 dB/km.Curvature losses are less than 10⁻³ dB/m.

The effective mode surface area is 89.8 μm².

Naturally, the present invention is not limited to the particularimplementations described above.

In particular, when precise geometrical shapes such as trapezium,triangles, rectangles, etc. are mentioned, it is clear that in practice,the profiles obtained may depart to a greater or lesser extent fromideal profiles, and it has been shown in the literature that suchdepartures, providing they are kept under control, do not alter theexpected properties of the optical fibers concerned. By way of example,FIG. 4 shows a real profile 40 of the trapezium and central ring type,as measured on an optical fiber. It is shown that the real profile 40 isequivalent to the ideal profile 41 also shown in FIG. 4. For greaterdetails on such equivalence, reference may be made to Hitachi's U.S.Pat. No. 4,406,518.

Also, it is clear that the central ring of the trapezium and centralring type of profile is not necessarily of the ideal rectangle shape,but may likewise have an ideal trapezium or triangle shape, and moregenerally it may in practice have any shape equivalent thereto.

Thus, for fibers of the invention, it is important that the geometricalparameters (distance from the axis of the fiber, index difference)should substantially satisfy the relationships of the invention, butthere is no need for the real shape of the curves to comply exactly tothe ideal geometrical shape of the profile under consideration.

Finally, any means may be replaced by equivalent means without goingbeyond the ambit of the invention.

We claim:
 1. A monomode optical fiber having substantially zerochromatic dispersion in the vicinity of 1.55 μm, comprising an opticalcore having:a central portion whose refractive index as a function ofdistance from the axis of said fiber varies between a minimum indexn_(s) and a maximum index n_(s) +Δn, where Δn is strictly positive, saidindex being represented by a curve that is substantially trapeziumshaped; a first layer surrounding said central portion, having an indexsubstantially equal to n_(s) ; and a second layer surrounding said firstlayer, having an index varying over the range n_(s) to n_(s) +hΔn, where0<h<1; and optical cladding surrounding said second layer and having anindex substantially equal to n_(s), said profile being defined by thefollowing geometrical parameters: a: total radius of the core, measuredat said second layer; x: ratio of the radius of said central portion toa, where 0<x<1; y: ratio of the radius of said first layer to a, wherex<y<1; and r: ratio of the small base to the large base of saidtrapezium, with 0≦r≦1; said fiber being characterized in that a and Δnare determined so that the chromatic dispersion of said fiber issubstantially zero at 1.55 μm and the cutoff wavelength λ_(c) of saidfiber is such that: 1.4 μm<λ_(c) <1.55 μm, r being selected arbitrarilyin the range 0 to 1, and then x, y, and h being selected so as tosatisfy the following relationships: ##EQU7##
 2. A fiber according toclaim 1, characterized in that a lies in the range 2 μm to 9 μm and Δnlies in the range 8×10⁻³ to 20×10⁻³.